Solvent separation of highly concentrated aromatic extracts



NOV 22, 1966 H. H. GRoss ETAL SOLVENT SEPARATION OF HIGHLY CONCENTRATEDAROMATIC EXTRACTS Filed March l, 1963 United States Patent 3,287,260SOLVENT SEPARATION OF HIGHLY CON- CENTRATED AROMATIC EXTRACTS Howard H.Gross, Pleasantville, N.Y., and Randlow Smith and Homer L. Spencer, Jr.,Houston, Tex., assiguors to Texaco Inc., New York, N.Y., a corporationof Delaware Filed Mar. 1, 1963, Ser. No. 262,116 8 Claims. (Cl. 208-321)This invention relates to the separation of liquid mixtures with aselective solvent. More particularly, it relates to the separation ofaromatic hydrocarbons from mixtures with other hydrocarbons by solventextraction with furfural, distillation of the resulting extract-solventmixture in the presence of the water vapor, and condensing andseparating a portion of the furfural in the absence of a liquid waterphase. In one embodiment of this invention, naphthalene and alkylnaphthalenes are separated from gas oils by selective solvent extractionwith furfural, the dissolved aromatics are concentrated and separated bya highly selective azeotropic distillation in the presence of furfuralemploying added steam, and a portion of the furfural is separatelycondensed and separately withdrawn in the absence of a liquid Waterphase.

Gas oils from some aromatic crude sources and gas oils from thermal andcatalytic cracking contain substantial quantities of useful aromatichydrocarbons, for example, benzenoid, naphthalene, and naphthalenichydrocarbons. This invention is particularly useful in the separation ofnaphthalene and naphthalene precursors, that is, naphthalenichydrocarbons which may be converted to naphthalene, for example byhydrodealkylation. The desirable aromatics are diluted with nonaromatichydrocarbons of the same boiling range making their separation by simpledistillation impossible. It is an object of this invention to separatearomatic hydrocarbons in concentrated form from mixtures withnonarom-atic hydrocarbons. Other objects will be apparent from thefollowing description and claims.

Solvent extraction is a well known process for the separation ofaromatic hydrocarbons from mixtures with nonaromatic hydrocarbons ofsimil-ar boiling range. Furfural has been found to be an excellentselective solvent in the separation of hydrocarbons of relatively highboiling point, for example, hydrocarbons luseful in the manufacture oflubricating oils and catalytic cracking feed stocks. Furfural extractionhas also been found useful in the manufacture of kerosene and lowboiling gas oil products where a rallinate of low aromatic content isproduced. However such lower boiling hydrocarbons, for example, thoseboiling within the range of 400 to 600 F. are considerably more solublein furfural than lubricating oils or cracking feed stocks. Attemperatures above about 100 F. both aromatic and nonaromatichydrocarbons boiling in the range of 400 to 600 F. are

relatively soluble and the selectivity for aromatic hydrocarbons isrelatively decreased. Although substantially complete separation ofaromatics from rainate streams may be readily effected, nonaromatics aredissolved in the extract to an extent that it is not possible toseparate extract fractions containing `aromatics in a concentrationabove about 80 percent at conditions readily obtained with ordinarycooling water. For the separation of aromatics in a concentration above80 percent, high solvent selectivity is necessary which in turn requirestemperatures far below usual ambient atmospheric temperatures, forexample, about 30 F. At these low temperatures the solvency of thefurfural is substantially reduced so that high solvent ratios, forexample, about 250 percent or higher are required to obtained acceptableyields.

vof the solvent and hydrocarbon phases.

Patented Nov. 22, 1966 In accordance with this invention, a liquidhydrocarbon mixture containing aromatic and nonaromatic hydrocarbons iscontacted with liquid furfural effecting formation of a raffinate mixcomprising a major portion of the nonaromatic hydrocarbons and anextract rnix comprising a furfural phase containing dissolvedhydrocarbons containing 70 to 85 volume percent aromatic hydrocarbonsand the remaining nonaromatic hydrocarbons. The extract mix containswithin the range of about 60 to 85 volume percent of furfural. At leasta part of said extract mix is distilled in a distillation'zone in thepresence of furfural and added water vapor (steam) effecting separationof an azeotropic mixture comprising Water, a major portion of thenonaromatic hydrocarbons present in the extract mix Vand a portion ofthe furfural as distillate. Aromatic rich hydrocarbons and remainingfurfural are separated as a bottoms fraction. Said bottoms are strippedseparating a furfural :stream in the absence of a liquid water phase andaromatic rich hydrocarbons.

In the extract mix azeotropic distillation step, water, preferably `assteam, is added at a rate within the range of about 0.05 to 3.0 poundsper pound of extract mix. The water carries a portion of the hydrocarbonoil into the rectification section of the distillation tower and thefurfural effects reduction of the relative volatility of the aromaticconstituents of the distilland so that nonaromatic hydrocarbonconstituents are concentrated in the distillate and aromatic hydrocarbonconstituents are concentrated in the bottoms. The distillation isconducted to remove within the range of about 3.0 to 30.0 volume percentof the furfural in the extract mix as distillate. The distillate, uponcondensation, separates into three phases, an oil phase, a water phaseand a furfural phase. The oil phase contains dissolved water 'andfurfural in an amount of about 0.1 to 0.5 and 1.0 to 5.0 volume percentrespectively. Similarly the water phase contains about 2.0 to 6.0 and8.0 to 10.0 volume percent dissolved oil and furfural respectively andthe furfural phase contains about 4.0 to 10.0 and 4.0 to 9.0 volumepercent dissolved oil and Water respectively. Solvent is readilyrecovered from the Water phase by azeotropic distillation and from theoil phase by water washing.

The solvent phase is treated to remove wa-ter and oil since theseconstituents interfere with the effectiveness of the solvent. Forexample, water in the solvent reduces its solvent power and maysepara-te in the extraction tower forming a third phase which interfereswith the contact Oil dissolved in the solvent phase is even moretroublesome since if not removed oil boiling near the boiling point ofthe solvent increases in concentration until an equilibrium is reachedand the solvent becomes ineicient in extracting'the. aromatic componentsof the feed. This dissolved oil and Water are concomitantly removed bydistillation with added steam to separte oil, water and furfuralazeotrope as distillate from furfural bottoms which are substantiallyIfree of dissolved oil and water.

ln accordance with this invention, the extract mix azeotropicdistillation step is effected with a minimum amount of steam andreboiler heat to effect separation of nonaromatic hydrocarbons overheadin an amount effective to increase the aromatic content of thehydrocarbon in the bottoms to the desired level but Without distillingoverhead more furfural than is necessary to form the azeotrope. In thismanner, a relatively large portion of the furfural remains in theextract mix. The remaining extract mix is then distilled without addingsteam to separate furfural from the aromatic concentrate. Since no steamis used in this nal distillation, a sharp separation between furfuraland oil is obtained and the furfural distillate contains less than`0.1volume percent dissolved water and less than 1.0 volume percentdissolved oil. By eifecting the separation of furfural from the extractmix in two steps of azeotropic distillation followed by a drydistillation, substantial savings in utilities consumption and equipmentcost are realized as compared with the separation of all of the furfuralin the azeotropic distillation. The two step separation of furfuralprovides a furfural stream which is substantially dry and oil-free and arelatively small amount of furfural phase requiring treatment forsepartion of oil and water whereas if all of the furfural is distilledoverhead in the extract mix azeotropic distillation column, all of thefurfural from the extract mix is collected in contact with water and oilphases and must be distilled a second time. Additionally, a greateramount of water phase and oil phase are formed in this redistillation.which must also be handled.

An .advantage of the process of this invention is that aromaticconcentrates containing more than 80 and preferably more than 90 percentaromatics may be separated from gas oil boiling range hydrocarbons withfurfural employing temperatures about usual ambient atmospherictemperatures and moderate solvent rates.

Another advantage of the process of this invention is that a singlesolvent is employed as both a selective solvent in an extraction stepand as a component of the azeotropic mixture in an azeotropicdistillation step.

Another advantage is that a stream of furfural is separated out ofcontact with a liquid water phase thereby producing a furfural streamcontaining relatively little oil and water and reducing the oil andwater content of this circulating solvent stream. g

The` accompanying drawing diagrammatically illustrates one form of theprocess of this invention. Although the drawing illustrates onearrangement of apparatus in which the process of this invention may bepracticed, it is not intended to limit the invention to the particularmaterials or apparatus described.

A gas oil feed is passed through line 1 into extraction tower 11. Intower 11 the oil feed is contacted with liquid tfurfural incountercurrent flow. The furfural solvent may be dry, that is,substantially free of dissolved water, or wet, that is, containing somedissolved water, as is well known in the art of solvent extraction withfurfural. Furfural introduced into tower 11 through* line 2 at a rate ofabout 100 to 300 Volume percent of the oil feed. In extraction tower 11aromatic components of the oil feed are preferentially dissolved in thefurfural and aromatic rich extract mix is withdrawn from the bottom oftower 11 through line 3. Rainate oil den-uded of aromatics is removedfrom the top of tower 11 through line 4 and discharged to raffinaterecovery facilities for removal of the small amount of dissolvedfurfural by conventional means. A temperature gradient of up to about 50F. and preferably within the range of about 10 to 30 F. is maintained intower 11. A desired temperature gradient may be maintained by heatingthe furfural passed to the top of the tower in exchanger 5 and coolingthe bottom of the tower with water in coil 6. Desirably the temperatureof the tower bottom is maintained as low as possible with cooling water,for example, about 80 to 100 F. When ambient atmospheric temperaturesand dew points are too high to obtain cooling water of desiredtemperature from a conventional cooling tower or spr-ay pond, thecooling water may be chilled by vacuum ashing, for example, water may becooled to 60 F. by discharge to a vacuum zone maintained at a pressureof 0.256 pounds per square inch absolute.

Extract mix is passed through line 3, preheater 16, and line 17 toextract-azeotrope tower 18. Tower 18 comprises a fractional distillationtower provided with a reboiler 20. Steam is introduced through line 21to assist in distillation and provide the water required'to satisfy theternary azeotrope composition of hydrocarbon, furfural, 'and water. Thisazeotrope is removed as overhead vapor from tower 18 through line 22.The hydrocarbon-waterfurfural azeotrope comprises a part nonaromaticcomponents and a part of the hydrocarbon extract and is distilled asoverhead product leaving the remaining furfural and aromatic hydrocarboncomponents of the extract as bottoms product. The presence of furfuralenhances the relative volatility of nonaromatic hydrocarbons as comparedwith the aromatic hydrocarbons so that the nonaromatic hydrocarbonsappear in the azeotropic mixture taken overhead and the aromatics areconcentrated in the bottoms. Aromatic concentrate and remaining furfuralare removed from the bottom of tower 18 through line 19.

Azeotrope vapors from tower 18 are discharged through line 22, cooled incondenser 25, and the resulting condensate is passed through line 26 toseparator 27. Separator 27 is provided with weirs 30 and 31. Condensatein separator 27 separates into three phases separated by interfaces 32and 33. The lower or heaviest phase is predominantly furfural andcontains a small amount of dissolved oil and water. The furfural layerhas an upper level at interface 33 and is confined by Weir 31. Furfurallayer is withdrawn through line 34 and a part is returned as reflux totower 18 through line 23. The intermediate phase is predominantly waterand contains a small amount of dissolved furfural and oil. The waterlayer has a lower level at interface 33- and an upper layer at interface32. The water layer overows weir 31 and is confined by weir 30. Waterphase is withdrawn through line 35. If desired, at least a portion ofthe reflux for tower 18 may comprise water from line 3S passed throughline 24 to line 23 thereby supplying a part of the Water required toform the azeotrope and reduce the amount of steam introduced throughline 21. The upper layer having a lower level at interface 32 ispredominantly oil and contains a small amount of dissolved furfural andwater. The oil phase overflows Weir 30 .and is withdrawn through line36. Oil withdrawn through line 36 is similar to the raffinate dischargedthrough -line 4 and may .be combined with it before or after separationof dissolved furfural.

Furfnral phase withdrawn through line 34 may be passed through lines 37and 38 to provide a portion of the solvent introduced into tower 11through line 2. Furfural in line 34 has a water content established bythe equilibrium at the temperature and conditions of separator 27. Wetfurfural in line 34 is passed through lines 37 and 40 to tower 41. Tower41 is provided with a reboiler 42. Dry furfural of reduced oil contentis withdrawn through line 43 and returned to line 38. Vapor comprisingfurfural, water and a small amount of oil is withdrawn through lines 44and 45, condensed in condenser 46, and the resulting condensate ispassed through line 47 to separator 50.

Water phase containing dissolved furfural can also be freed of furfuralby distillation since the azeotrope of water and furfural is lowerboiling then either furfural or water. In this case an azeotropecontaining all of the furfural and some of the water is removed overheadfrom all remaining furfural-free water which is withdrawn as bottoms.Water phase from separator 27 is withdrawn through lines 35 and 51 towater distillation tower 53. Steam is introduced into tower 53 throughline 54. Water free of furfural is discharged from the bottom of tower53 through line 49. Overhead vapor from tower 53, comprising water,furfural, and a small amount of oil, is discharged through line 55 andcombined with the similar mixture n line 44 from tower 41 forcondensation Iand separation.

The combined condensates from towers 41 and 53 are passed to separator50. Separator 50 is similar in design and function to separator 27.Furfural phase having an upper level at interface 60 and confined byWeir 57 is withdrawn through line 61 and combined with the turfuralphase in line 34 for eventual recycle to extraction tower 11. Waterphase bet-Ween interfaces 60 and 62 overows weir S7 but is confined byWeir 56. Water phase is withdrawn through line 63 and combined with thewater from line l35 for furfural separation in water distillation tower53. Oil layer separating above interface 62 overows .Weir 56 and iswithdrawn through line 64. This oil may be combined with oil in line 36or 4 for furfural separation and recovery, not shown.

'The aromatic concentrate-furfural stream Withdrawn from tower 18through line 19 is passed to dry furfural stripping -tower 70. Tower 70is provided with a reboiler 72 to distill remaining furfural out of thearomatic concentrate which is withdrawn from the bottom of tower 70through line 71. yOverhead vapor comprising furfural is dischargedthrough line 7-3 to condenser 74. Condensed furfural liquid is passe-dthrough line 75 to receiver 76. Dry furfural, that is, furfuralsepara-ted in the absence of a separate liquid water phase, is withdrawnthrough line 80 and a portion [passed through line 81 to tower 70 as areflux. Remaining dry -furfural is passed through line 82 to line 38-for recycle through heater 5 and line 2.

High extraction yields, for example, in excess of about 90 percent of-the aromatics contained in the gas-oil feed are obtained in -theextraction step of this process. High extraction yields lare favored byhigh solvent ratios and relatively high extraction temperatures. At highextraction yield levels, because of the lower solvent selectivity atconditi-ons favoring high yields, a substantial amount of nonaromatichydrocarbons is also extracted so that the concentration of aromatics inthe extracted hydrocarbon is at least 70 percent and preferably -withinthe range of about 80 to 85 percent. Since ymost of the aromatics areincluded in the extract, the rafiinate contains very little aromatichydrocarbon and is therefore particularly valuable in -t-he manufactureof heating oils, diesel fuel oils, catalytic cracking feed stocks andother products Where aromatics are undesirable.

In the azeotropic -distil'lation step, the concentration of aromatics inthe hydrocarbon portion of the bottoms is increased to about 90 percentor higher |by selectively removing nonaromatic hydrocarbons. Theazeotnopic distillation is effected at a top tower temperature of about270 to 300 F. and with a bottom tower temperature of about 400 to 550 F.Steam is added at Ithe bottom of the aze-otropic distillation tower at arate of about 0.05 to 3.0 pounds per pound of extract mix to satisfy thecomposition lrequired to form the azeotrope and to assist in theseparation of nonaromatic hydrocarbons from the extract bottoms product.Furfural rich phase, Water rich phase or both, separated from thecondensed azeotrope distilled overhead, are returned to the azeotropicdistillation tower as reflux to provide rectification of thehydrocarbons rising i-n the tower eilecting enrichment in aromaticconstituents of the down-owi-ng liquid.

Example In accordance with the process of this invention, a gas oilproduced by catalytic cracking is treated for recovery of aromatics. Thecatalytic cracked gas oil charge stock has the followingcharacteristics:

Gravity, API 30.0 Distillation, ASTM, IBP, F 400 Distillation, ASTM, EP,F. 550 Hydrocarbon analysis, percent by vol.

Aromatics 55.0 Parains 34.0

Naphthenes 11.0

The gas oil charge is contacted at a rate of 2500 barrels per day(bp-d.) with furfural solvent in a countercurrent contactor at a solventdosage of 200 volume percent. The furfural solvent contains 5.0 volumepercent dissolved water and 3.0 volume percent dissolved oil. Atemperature gradient of 50 F. is maintained in the contactor with thetop temperature (ranate) of 130 F. and bottom temperatureextract) of 80F. Rainate mix is stripped to separate dissolved furfural and produce arainate produc-t in a yield of 46.2 volume percent of the charge oil.Extract mix is produced at a rate of 6,445 bpd. containing 75.9 volumepercent -furfural and 24.1 volume percent hydrocarbon, the aromaticcontent of which is volume percent.

Extract rnix at a ra-te of 6,445 b.p.d. is passed to an azeotropicdistillation column together with 143,400 pounds per day of steam. Anazeotrope is distilled over- 'head containing furfural, steam andhydrocarbon and bottoms are withdrawn containing 77.5 volume perce-ntfurfural and 2.2.5 volume percent `hydrocarbon havin-g an aromaticcontent of 93 volume percent. Distillate is condensed separating 170bpd. of oil phase, 460 btap.d. of water phase and 251 bpd. of futnfuralphase. `Bottoms from .the azeotropic `distillation column are passed toa second distillation column at a rate of 5,975 b.p.d.

Furfural containing substantially no water and 1.0 volume percent oil ata rate of 4,630 b.p.d. is distilled overhead an-d aromatic concentratecontaining 93 volume percent aromatics is withdrawn at a rate of 1,345bpd. as bottoms product.

We claim:

1. A method for the separation of an aromatic rich fraction from aliquid hydrocarbon mixture which comprises:

contacting said mixture with a solvent comprising liquid furfural in anextraction zone separating a rafiinate mix comprising a hydrocarbonphase containing dissolved furfural and lan extract mix comprising afurfural phase containing dissolved hydrocarbons containing 70 to 85volume percent aromatic hydrocarbons,

distilling at least a part of said extract mix in a first distillationzone in the presence of added steam in an amount within the range of0.05 to 3.0 weight percent of said extract mix separating a firstdistillate comprising water, nonaromatic hydrocarbons and furfural andla Iirst bottoms liquid comprising turfural and hydrocarbons containingat least volume per cent aromatic hydrocarbons, separating said firstdistillate into a hydrocarbon-rich phase comprising at least a portionof said non-aromatic hydrocarbons, a water-rich phase, and afurfural-rich phase comprising furfural containing dissolved oil andwater,

separating at least a portion of said dissolved oil and Water from saidfurfural-rich phase forming a treated furfural-rich phase,

passing said treated furfural-rich phase to said extraction zone as aportion of said solvent,

distilling at least a part of said rst bottoms liquid in a seconddistillati-on zone separating a second distillate comprising furfural inthe absence of a liquid water phase and a second bottoms liquidcomprising said hydrocarbons containing at least 90 volume percentaromatic hydrocarbons said second distillate containing less than 0.1volume percent dissolved Water and less thna 1.0 volume percentdissolved oil, and

passing said second distillate to said extraction zone as a portion ofsaid solvent.

2. The process of claim 1 wherein said liquid hydrocarbon mixture is agas oil produced by catalytic cracking.

3. The process of claim 1 wherein the extract mix from said extr-actionzone contains within the range of 65 to 85 volume percent furfural.

4. The proces of claim 1 wherein the furfural in said first distillateis within the lrange of 3.0 to 30.0 volume percent of the furfural insaid extract mix.

5. In a method for the separation of an aromatic fraction containing atleast 90 percent by volume of aromatic hydrocarbons from a catalyticcracked gas oil wherein said gas oil is contacted with a liquid solventcomprising furfural at a solvent -ratio -within the range of about to300 volumes of solvent to 100 volumes of said gas oil in a solyentextraction zone, and an extract mix comprising furfural and dissolvedhydrocarbons containing 70 to 85 volume percent aromatic hydrocarbons iswithdrawn from said extraction zoneytheimprovement which comprises:

distilling at least a portion of said extract mix in a rst distillationzone in contact with added steam in an amount within the range of 0.05to 3.0 weight percent of said extract mix separating a rst distillatecomprising an azeotrope of Water, furfu'ral and hydrocarbons from a rstbottoms liquid comprising furfural and hydrocarbons containing at least90 percent aromatic hydrocarbons, by volume, separating said rstdistillate intor a hydrocarbon-rich phase comprising at least a portionof said nonaromatic hydrocarbons, a Water-rich phase, and afurfural-rich phase comprising furfural containing dissolved oil andwater, Y

separating at least a portion of said dissolved oil and Water from saidfurfural-rich phase forming a treated furfural-rich phase, A

passing said treated furfural-rich phase to said extraction zone as aportion of said solvent,

distilling said rst bottoms liquid in .la secondvdistillation zoneseparating a second distillate comprising furfural in 4the absence of aliquid water phase and containing less than 0.1 volume percent dissolvedwater and 1.0 volume percent dissolved oil and a second bottoms liquid4comprising said aromatic '8 fraction containing at least percent byvolume of aromatic hydrocarbons, and

passing said second distillate to said extraction zone as a portion ofsaid solvent.

6. The method of claim 5 wherein said solvent extraction zone ismaintained with a top temperature of about V F. and a bottom temperatureof about 80 F.

7. The method of claim 5 wherein said catalytic cracked gas oil boilsWithin the range of about 400 to 550 F. by ASTM distillation.

8. The method of claim 5 wherein said first distillation zone ismaintained at a top tower temperature within the range of about 270 to300 F. and at a bottom tower temperature within the range of about 400to 550 F.

References Cited by the Examiner UNITED STATES PATENTS 2,534,383 12/1950Arnold et al 208-321 2,567,172 9/1951 Arnold et al 208-313 2,878,2613/1959 Broughton 208-321 2,886,610 5/1959 Georgian 208-321 2,895,9087/1959 Beavon 208-321 DELBERT E. GANTZ, Primary Examiner. ALPHONSO D.SULLIVAN, Examiner.

H. LEVINE, Assistant Examiner.

1. A METHOD FOR THE SEPARATION OF AN AROMATIC RICH FRACTION FROM ALIQUID HYDROCARBON MIXTURE WHICH COMPRISES: CONTACTING SAID MIXTURE WITHA SOLVENT COMPRISING LIQUID FURFURAL IN AN EXTRACTION ZONE SEPARATING ARAFFINATE MIX COOPRISING A HYDROCARBON PHASE CONTAINING DISSOLVEDFURFURAL AND AN EXTRACT MIX COMPRISING A FURFURAL PHASE CONTAININGDISSOLVED HYDROCARBONS CONTAINING 70 TO 85 VOLUME PERCENT AROMATICHYDROCARBONS, DISTILLING AT LEAST A PART OF SAID EXTRACT MIX IN A FIRSTDISTILLATION ZONE IN THE PRESENCE OF ADDED STEAM IN AN AMOUNT WITHIN THERANGE OF 0.05 TO 3.0 WEIGHT PERCENT OF SAID EXTRACT MIX SEPARATING AFIRST DISTILLATE COMPRISING WATER, NONAROMATIC HYDROCARBONS AND FURFURALAND A FIRST BOTTOMS LIQUID COMPRISING FURFURAL AND HYDROCARBONSCONTAINING AT LEAST 90 VOLUME PERCENT AROMATIC HYDROCARBONS, SEPARATINGSAID FIRST DISTILLATE INTO A HYDROCARBON-RICH PHASE COMPRISING AT LEASTA PORTION OF SAID NON-AROMATIC HYDROCARBONS, A WATER-RICH PHASE, AND AFURFURAL-RICH PHASE COMPRISING FURFURAL CONTAINING DISSOLVED OIL ANDWATER, SEPARATING AT LEAST A PORTION OF SAID DOSSOLVED OIL AND WATERFROM SAID DURFURAL-RICH PHASE FORMING A TREATED FURFURAL-RICH PHASE,